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Stradivari's golden ground varnish


Nicolaus
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I now have had time to read The Strad article and like it. The researchers corroborate other work detecting burnt (calcined) bone and pumice as varnish driers. This could be an explanation of the rubble detected in some varnish systems. 

I really like how they presented the ancient varnish references by listing them chronologically. This makes a strong case for the researchers.

However, the varnish driers have only a slight effect on what we see today because they are now practically invisible. This is only part of the saga of Cremonese varnishing.

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39 minutes ago, Michael_Molnar said:

I now have had time to read The Strad article and like it. The researchers corroborate other work detecting burnt (calcined) bone and pumice as varnish driers. This could be an explanation of the rubble detected in some varnish systems. 

I really like how they presented the ancient varnish references by listing them chronologically. This makes a strong case for the researchers.

However, the varnish driers have only a slight effect on what we see today because they are now practically invisible. This is only part of the saga of Cremonese varnishing.

Unfortunately I haven't read The Strad article (I'm not a subscriber) but it's the first time I've heard of burnt bones and pumice. Each new research finds something different, what's the next?

But it is understandable, if there were not some original news it would be boring and no one would read their articles.:D

But I like this new research, because I use pumice, and I must have some bone meal from Kremer somewhere...

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I also read the article and found it quite interesting, with my limited understanding of varnish or anything else for that matter... What I wonder though is if pumice is detected because it was incorporated directly into the varnish, or would it be the remains of polishing or smoothing layers before applying the next?

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23 hours ago, David A.T. said:

I used potassium silicate for the first instruments I made.

Pure potassium silicate in crystalline form is colorless. But when ground into a powder, it will appear white because the irregular surfaces of the powder grains will scatter light.

But this silicate also has a refractive index of 1.5+, about the same as wood, linseed oil and resin based oil varnishes. So when these varnishes are applied to the white powder, it will stop scattering the light and the texture and color of the wood will reappear.

If varnishes are penetrating into the wood after you applied this ground, then you most likely used too much oil and/or not enough silicate. You want just enough oil to make a very thick paste so you can evenly spread the thinnest of layers of the silicate onto the wood. Once it dries, it should be clear unless you did not use enough oil to cover all the powder grains.

Besides the challenge of getting the right amount of oil to silicate powder, there is also the challenge of getting the grain size fine enough to penetrate the openings in the wood grain. Mineral ground that simply sits on the surface of the wood can have adhesion problems. Additional layers of varnish may fracture or peel off.

A downside to this silicate is that it is highly soluble in water. Anything applied to wood that also reacts with water is asking for trouble in humid conditions. But it also supplies another option for applying it.

Create a clear, near saturated solution of the silicate in water. Now gently dab the solution onto the wood and let it dry between additional coats. You can finish by gently rubbing the surface to get a uniform white coat. This will guarantee that the silicate will penetrate into the wood grain and thoroughly seal it. Now apply oil varnish as usual to restore the transparency.

Be aware that water-based ground applied this way can cause the grain to swell in visually undesirable ways.

The challenge with pumice as a mineral ground is that it is a mixture of different colored minerals. It is normally not transparent. So when applied to wood as a fine powder, you are essentially coloring the wood, which means grain detail can be obscured. If you can grind it into a very fine powder, and only apply enough to penetrate into the wood pores without sitting atop the wood itself, you can both seal the wood and perhaps introduce an interesting colored pattern into the wood.

 

 

 

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Potassium silicate.

There are references in old cabinetmaker"s documents about horsetail as a polishing tool.  It was not used flat like sandpaper.

The reeds were bundled like a small broom.  The ends of the reeds were scored and frayed with a knife.   The bundle was the soaked in "weak barber's acid".  Then the broom was used to polish bare wood.

Barber's acid is a weak lye solution.  Silicate from the horsetail + potassium from the lye solution = potassium silicate.

on we go

Joe

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16 minutes ago, joerobson said:

Potassium silicate.

There are references in old cabinetmaker"s documents about horsetail as a polishing tool.  It was not used flat like sandpaper.

The reeds were bundled like a small broom.  The ends of the reeds were scored and frayed with a knife.   The bundle was the soaked in "weak barber's acid".  Then the broom was used to polish bare wood.

Barber's acid is a weak lye solution.  Silicate from the horsetail + potassium from the lye solution = potassium silicate.

on we go

Joe

Isn't lye sodium hydroxide though? Or did they use potassium in the past?

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Friends, you are going down a rabbit hole by not having read the article. :wacko:

However, I did find their original research paper:

Stefan Zumbuhl, Balthazar Soulier, Christopher Zindel. ”Varnish technology during the 16th - 18th century: The use of pumice and bone ash as solid driers.” Journal of Cultural Heritage 47 (2021) 59-68.

I subscribe to ScienceDirect.com to read this.

 

 

 

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17 minutes ago, Michael_Molnar said:

Friends, you are going down a rabbit hole by not having read the article. :wacko:

Actually, I have somewhere a research paper proposing burnt bones and pumice. I’ll try looking for it later. Maybe, someone else can find it.

 

I would like to see that paper considering I don't have a subscription to the Strad.  Hopefully you can find it.   What is meant by burnt bone?  Wouldn't that just turn bones into carbon?  

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5 minutes ago, MikeC said:

I would like to see that paper considering I don't have a subscription to the Strad.  Hopefully you can find it.   What is meant by burnt bone?  Wouldn't that just turn bones into carbon?  

The Strad would rightfully have a fit with people bypassing their subscription. Google “bone ash”. Also, look into sciencedirect.com and Elsevier Masson.

 

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4 hours ago, ctanzio said:

Once it dries, it should be clear unless you did not use enough oil to cover all the powder grains.

 

Thanks for your Xp and advices.

The first time I used silicate (diluted 28% to 50% which makes about 14% solution) I got this :

image.thumb.png.15fbb5c76024fc450c15340c903a458d.pngimage.thumb.png.c4afcde01f6f4991b972b8d9a75a25a9.png

I think I put Lime water on it (but not sure - I forgot) because it was recommended in these video :

https://www.youtube.com/watch?v=Tzq4VlE5svY

according to the Luthier it makes it insoluble Silicate

 

Then I removed all the silicate/calcium at the surface and applied a clear varnish.

 

after I got this, which was fine to me.

image.thumb.png.94e084f19c9160829d6366231f025e46.png

(same as in museum, the light, the angle, and the time of the picture helps a bit for the gold rendering)

4 hours ago, joerobson said:

Potassium silicate.

There are references in old cabinetmaker"s documents about horsetail as a polishing tool. 

For the polishing I tried this horsetail from a local supplier :

image.png.3bbaecd4c6d37d9c13b5ecb653f072be.png

it was not easy to use in fact because it goes everywhere on the plates and inside the f holes when polishing with that. I did not notice better effect than a fine sand paper.

Then finally I had a not a too bad experience with Silicate and Clear varnish. I did not notice issue with humidity so far.

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46 minutes ago, David A.T. said:

For the polishing I tried this horsetail from a local supplier :

image.png.3bbaecd4c6d37d9c13b5ecb653f072be.png

it was not easy to use in fact because it goes everywhere on the plates and inside the f holes when polishing with that. I did not notice better effect than a fine sand paper.

I am not surprised you had trouble using this, because it appears to be all crushed up.  I haven't seen it this way before.  Is there some reason not to use the segments intact?  

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15 hours ago, joerobson said:

No....but the potassium is provided by the corrosive action of the lye.

And I think it is due to potassium caseinate.

Invernizzi, C.; Fiocco, G.; Iwanicka, M.; Targowski, P.; Piccirillo, A.; Vagnini, M.; Licchelli, M.; Malagodi, M.; Bersani, D. Surface and Interface Treatments on Wooden Artefacts: Potentialities and Limits of a Non-Invasive Multi-Technique Study. Coatings 2021, 11, 29.

https://doi.org/10.3390/coatings11010029

 

 

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45 minutes ago, Michael_Molnar said:

And I think it is due to potassium caseinate.

Invernizzi, C.; Fiocco, G.; Iwanicka, M.; Targowski, P.; Piccirillo, A.; Vagnini, M.; Licchelli, M.; Malagodi, M.; Bersani, D. Surface and Interface Treatments on Wooden Artefacts: Potentialities and Limits of a Non-Invasive Multi-Technique Study. Coatings 2021, 11, 29.

https://doi.org/10.3390/coatings11010029

From what I've gathered reading various things I've come to the same conclusion. I do wonder though if there's more than one answer... maybe there's several contributing factors?

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I haven't read all 13 pages, so please forgive me if I'm wandering too far off base or treading ground already trod. I think the "gold" notion is overblown and has become mythicalized. The color of the wood on classical Cremonese instruments I've seen is not what I would call "gold", but rather a more subdued and complex blend of yellows, greys, greens, and other hues. 

I also don't think this "golden ground" was a varnish, but rather the result of pre varnish wood surface treatments. Look at Joe Robson's ground system and aged wood colors as an example. 

Bruce Tai et al's recent paper also discusses chemical pre varnish wood treatments, some of which would certainly affect wood color. 

I use something akin to a synthetic Roubo primer, followed by a relatively colorless ground varnish. Cello by Kile Hill, primer and ground by me.

IMG_20210917_150131_767.jpg

kilehillcellos_1631922952579.jpg

IMG_20210910_190128.jpg

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7 hours ago, JacksonMaberry said:

I haven't read all 13 pages, so please forgive me if I'm wandering too far off base or treading ground already trod. I think the "gold" notion is overblown and has become mythicalized. The color of the wood on classical Cremonese instruments I've seen is not what I would call "gold", but rather a more subdued and complex blend of yellows, greys, greens, and other hues. 

I also don't think this "golden ground" was a varnish, but rather the result of pre varnish wood surface treatments. Look at Joe Robson's ground system and aged wood colors as an example. 

Bruce Tai et al's recent paper also discusses chemical pre varnish wood treatments, some of which would certainly affect wood color. 

I use something akin to a synthetic Roubo primer, followed by a relatively colorless ground varnish. Cello by Kile Hill, primer and ground by me.

IMG_20210917_150131_767.jpg

kilehillcellos_1631922952579.jpg

IMG_20210910_190128.jpg

Looks a lot like what I have been doing, but colleagues are telling me it is too bright and brassy. I have another idea that throttles it back a tad.

Stay tuned.

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